Conventional water heating devices comprise an outer cylindrical structure or casing, an inner cylindrical structure coaxially supported in the casing to define an outer water-flow passage between the two cylindrical structures, and an inner water-flow passage within the inner structure, the outer and inner flow passages being in communication with each other at one end of the casing. The inner structure comprises a cylindrical support formed of ceramic and secured at one end to one end of the casing, a resistance heating element on the outer surface of the cylindrical support and a ceramic sheet in which the heating element is embedded. The surface temperature of the inner structure, or heater is determined by the relative thicknesses of the ceramic support and sheet and the heat transfer coefficient to water on the inner and outer surfaces of the heater. Since the water flows in the inner flow passage at a speed higher than it flows in the outer flow passage, the heat transfer coefficient is greater at the inner wall of the heater than at its outer wall. As a result, the heater has a greater thermal resistance on the inner surface than on the outer surface due to the larger thickness of the cylindrical support. Therefore, the temperature at the inner wall of the heater is higher than the temperature at the outer surface and the difference between them is as large as 40.degree. C. Such temperature differences result in unbalanced heat transfer conditions, so that the entire surface area of the heater is not effectively utilized to transfer thermal energy. Furthermore, the outer surface temperature tends to rise excessively so that the water is boiled at localized areas and the main substances of the scales formed on the outer surface, such as calcium bicarbonate and magnesium bicarbonate, are dissolved and precipitate on the outer surface of the heater. Such precipitation causes the surface temperature to increase abnormally to the extent that the resistance element is broken.